DD 12 in year 7 hates leaving anyone out and obviously doesn't like it much when she is left out although I tell her it's normal to not be invited to everything.
She has made a group of friends, there are 7 of them. They hang out at school and have had other meet ups as a group (not always the same girls). Dd likes all her friends but she is closer to 3 of them.
She isn't enjoying the big group stuff much at the moment as the girls have started egging each other on and it gets a bit full on, typical for this age. Dd would like to invite 4 of the group, which means 2 are left out so she is inviting 3 but leaving out a girl she really likes and wants to get to know better.
YABU = Invite the 4 you like and don't be too concerned about the 2 that are left out as dd not that close with them (still not nice for them)
YANBU = invite only 3 so 3 are not invited rather than 2.
Alternative is to invite only 3 (half the group), but arrange something with the girl she likes who isn't invited separately. This girl is lovely and dd likes her a lot and dd has been invited to her house a few times.